The two types of variables that we can choose from are 1) the Independent Variable and 2) the Dependent Variable.
The independent variable is one that you can change freely, whereas the dependent variable is the variable that changes depending on what another variable's (independent variable) value is.
So in this example given, the independent variable is the price of the hotdog, since the number of hotdogs sold, being the dependent variable, varies directly with the price of these hotdogs.
Therefore:
Independent Variable: Price of the hotdog
Dependent Variable: Number of hotdogs sold
